Somebody on the Steam L4D forums discovered that if you reset your lamb 2 slaughter achievement and then earn it again it'll give it to you on your steam community page. Just type this in the console and then attempt (because nobody is stupid enough to leave the safe room once they're in, right? right?) to re-earn the achievement:

Anyway, I resetted it... but I probably gain it through legit means later. >_>Edit: Expect you need to gain it before restarting L4D. Oh well, guess I just fake it with bots to have it register properly. I had it since "before".